Nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A device equipped with a lithium ion battery includes a wide variety of electrical protection circuits, provided in a battery charger or a battery pack, so that the battery operates normally and safely. However, if a breakdown or a malfunction, for example, occurs in the protection circuits, the lithium ion battery may be continuously recharged. This may cause oxidative and reductive degradation of an electrolyte on surfaces of a positive electrode and a negative electrode both of which generate heat, oxygen release caused by decomposition of a positive electrode active material, and even deposition of metallic lithium on the negative electrode. This may eventually cause the lithium ion battery to fall into a thermal runaway. There is also a danger that ignition or burst may occur in the lithium ion battery, depending on the situation.
In order to safely stop a battery before such a dangerous thermal runaway occurs, most lithium ion batteries currently include, as a separator, a porous base material which contains a polyolefin as a main component and which has a shutdown function of clogging pores present in the porous base material when a temperature inside the battery is raised due to some defect and reaches approximately 130° C. to 140° C. The shutdown function is exhibited when a temperature inside the battery rises. This stops passage of ions through the separator, and thus allows the battery to safely stop.
Meanwhile, a porous base material which contains a polyolefin as a main component melts when the porous base material is exposed to a temperature equal to or higher than a temperature at which the shutdown function works. This is because heat resistance of such a porous base material is low. This may cause a short circuit inside a battery and may consequently lead to ignition or explosion of the battery. In view of the above, with an aim to improve the heat resistance of the porous base material, efforts are being made to develop a separator that includes a porous layer containing a filler and a resin on at least one surface of the porous base material.
For example, Patent Literature 1 discloses a nonaqueous secondary battery separator arranged such that a porous layer, containing a plate-shaped inorganic filler and having a porosity of 60% to 90%, is provided on at least one surface of a porous base material. Further, Patent Literature 1 discloses that the occurrence of a short circuit can be prevented by setting a degree of orientation of the plate-shaped inorganic filler in a specific range. The degree of orientation of the plate-shaped inorganic filler is calculated from an azimuth-dependency of scattering peak intensity in an X-ray diffraction, in which X-rays are incident along a cross sectional direction of the separator and perpendicularly to a cross section of the separator.